Conventionally, for the application of a semiconductor element such as a light-emitting element or a light-receiving element, a semiconductor wafer which is obtained by epitaxially growing a III-V system compound semiconductor layer, for example, InP, on an InP substrate, has been widely used. The epitaxial layer of the compound semiconductor is formed, for example, by the metalorganic chemical vapor deposition method (hereinafter referred to as MOCVD method).
When the above-described III-V system compound semiconductor layer is epitaxially grown by the MOCVD method, minute convex defects called hillock and wrinkle defects called orange peel are caused on the surface of the epitaxial layer. As a result, a problem of deterioration of the surface morphology of the epitaxial layer, was caused. Thus, various techniques for improving the surface morphology of the epitaxial layer, have been suggested.
For example, according to Patent Publication 1, a method in which a wafer of which a plane orientation is inclined from a <100> direction by an angle of 0.1° to 0.2° is used as a substrate for epitaxial growth by the MOCVD method and a crystal is epitaxially grown at a substrate temperature of 600° C. to 700° C., has been suggested. This method is successful in remarkably reducing hillocks (called as tear-shaped defect in Patent Publication 1) in the surface of an epitaxial layer.
Furthermore, in Patent Publication 2, an epitaxial growth method in which the range of an off angle of a substrate is defined by a function of a growth rate and a substrate temperature, has been suggested in order to prevent orange peel from being caused when the off angle of is increased. By this method, hillocks caused in the surface of an epitaxial layer can be significantly reduced and the generation of orange peel can be prevented.
Furthermore, in Patent Publication 3, a method in which an off angle of a substrate is defined by additionally considering a defect density (dislocation density) of the substrate has been suggested. Specifically, when a thin film of a compound semiconductor is epitaxially grown on an InP substrate, the substrate having an off angle θ(°) from the <100> direction which satisfies a condition of θ≧1×10−3 D1/2 (D(cm−2): defect density of the substrate) is used. For example, when a defect density D of the substrate is 1000 cm−2, the substrate having an off angle of θ≧0.03° is used. When a defect density D of the substrate is 1000 cm−2, the substrate having an off angle of θ≧0.10° is used.
Furthermore, the present applicant pays attention on the micro roughness of a surface of an epitaxial layer and suggests a technique that by obtaining haze of 2 ppm or less over the entire effective use region of the substrate and by obtaining an off angle of the substrate of 0.05 to 0.10°, the surface of the epitaxial layer can be improved at a level of micro roughness (Japanese Patent Application No. 2004-079504). The term “haze” herein is defined by a value obtained by dividing the intensity of scattering light obtained when light from a predetermined light source enters the surface of the substrate by the intensity of incident light from the light source.
[Patent Publication 1] Japanese Patent No. 1975143
[Patent Publication 2] Japanese Patent No. 2750331
[Patent Publication 3] Japanese Patent No. 3129112